Flat tube for heat exchanger

ABSTRACT

A flat tube having a nearly B-like shape or the like in cross section, which has one or more partitions therein, and is coated with a brazing metal on the outer surface thereof, is provided. In the top portion of the turned-up portion constituting the partition, there are formed many slits, in order to provide the conditions that satisfy both of the brazing performance and an increase of working accuracy of the flat tube, which slits allow the brazing metal to enter up to the top portion of the turned-up portion, wherein the length “c” of the slit is 2 mm to 15 mm; the distance “e” between the edges of the neighboring slits is 3 mm to 10 mm; and “e/c” is 0.6 or more.

TECHNICAL FIELD

The present invention relates to an aluminum-made flat tube for heatexchanger having one or more partitions therein, of which cross sectionis formed in a nearly B-like shape or the like, in particular to analuminum-made flat tube for heat exchanger, which is formed by bending astrip-shaped metal plate coated with a brazing metal at the outersurface side thereof in the width direction and formed with slits in atop portion of the partition for allowing the brazing metal at the outersurface side to enter into the inner surface side through the slits forbrazing the partition and the inner wall surface.

BACKGROUND ART

There is known a flat tube having a nearly B-like shape or the like insection, in which slits are intermittently formed in a top portion of apartition positioned in the central area thereof so as to allow abrazing metal at the outer surface side of the tube to enter into theinner surface side through the slits while brazing to integrally fix thetop portion of the partition and the opposed inner surface of the tubeby means of brazing; thereby the performance against the pressure isincreased (for example, refer to FIGS. 8 and 9 in Japanese PatentApplication Laid-open No. 2002-228369).

When a corrosive fluid is circulated inside an aluminum-made flat tube,the inner surface side of the core metal of the flat tube is clad with asacrificial anode material, and the outer surface side is clad with abrazing metal. In a flat tube having a nearly B-like shape in section,when a partition at the central area is formed in a manner of turn-upbending, the top portion and the inner side of the tube abutting thereonhave to be brazed. In such case, the brazing metal at the outer surfaceside of the tube is allowed to enter into the inner surface side throughslits formed in the top portion thereof.

However, from experiments conducted by the inventor, the following factwas found; i.e., in the flat tubes using slits, the reliability of thebrazing largely varies depending on the length of the respective slits,gaps between the slits and the like, and the workability into a tube ofB-like shape in section and accuracy thereof is largely influencedthereby.

Accordingly, an object of the present invention is to determineexperimentally optimum conditions for the slits formed in the topportion of the partition in a flat tube having one or more partitionsand formed into a B-like shape in section.

DISCLOSURE OF THE INVENTION

An aspect of the present invention, disclosed in claim 1, is a flat tubefor heat exchanger, which includes:

a pair of flat face portions (1) parallelly opposed to each other and apair of curved portions (2) connected to both ends of the flat faceportions (1) formed with a strip-shaped metal plate bent in the widthdirection thereof to form into a flat cylindrical shape, wherein thestrip-shaped metal plate is coated with a brazing metal (3) on onesurface thereof, and is bent so that the brazing metal (3) is positionedat the outer surface side of the cylindrical shape;

in the central position in the width direction of one of the flat faceportions (1), a turned-up portion (4) is bent up to the opposed flatsurface side, and the top portion (5) of the turned-up portion (4) abutson the inner surface of the opposed surface side to form a partitionwithin the tube;

many slits (6) for allowing the brazing metal to enter therethrough areformed intermittently being separated away from each other in the topportion (5) in the longitudinal direction thereof,

-   -   wherein the length “c” of the slit (6) is 2 mm to 15 mm; the        distance “e” between the edges of the neighboring slits (6) is 3        mm to 10 mm; and “e/c” is 0.6 or more.

Another aspect of the present invention, disclosed in claim 2, is theflat tube for heat exchanger according to claim 1, wherein the thicknessof the strip-shaped metal plate is 0.15 mm to 0.6 mm.

The flat tube for heat exchanger according to the present invention hasa structure as described above, and provides the following effects.

The flat tube for heat exchanger according to the present invention isstructured so that the top portion 5 of the turned-up portion 4, whichis formed in the central portion in the width direction of the flat faceportion 1, abuts on the inner surface at opposite side thereto to form apartition in the tube, wherein many slits 6 are formed intermittentlybeing separated away from each other in the top portion 5, and thelength of the slits 6 is 2 mm to 15 mm; the distance between the edgesof the neighboring slits 6 is 3 mm to 10 mm; and “e/c” is 0.6 or more.Accordingly, a highly reliable flat tube for heat exchanger having asatisfactory brazing strength between the top portion 5 and the innersurface at the opposite side, which provides a high performance againstthe pressure and which generates no deformation nor twist while the flattube is being formed, is provided.

That is, since the length of the slits 6 is prescribed to 2 mm or more,the brazing metal reliably enters to the inner surface side through theslits 6 while brazing, the reliability on the brazing is ensured.

Since the length of the slits 6 is prescribed to 15 mm or less, theworking accuracy to form the turned-up portion 4 by bending thestrip-shaped metal plate in the width direction thereof can be highlymaintained; as a result, the reliability on the flat tube for heatexchanger can be maintained.

Also, since the distance between the edges of the neighboring slits 6 isprescribed to 3 mm or more, no crack is generated between the edges ofthe slits 6, a highly reliable flat tube can be provided.

Further, since the distance between the edges of the neighboring slits 6is prescribed to 10 mm or less, the fillet of brazed point in the topportion 5 is formed satisfactorily while brazing, a flat tube for heatexchanger with high strength and performance against the pressure can beprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross sectional view of a flat tube for heat exchangeraccording to the present invention, illustrating a relevant portion of afirst embodiment.

FIG. 2 shows a cross sectional view illustrating the state of use of theflat tube after brazing.

FIG. 3 shows a perspective view schematically illustrating a turned-upportion 4 of the flat tube.

FIG. 4 shows an illustration of a strip-shaped metal plate before beingshaped into the turned-up portion 4 of the flat tube.

FIG. 5 shows a cross sectional view illustrating a relevant portion of aflat tube for heat exchanger in a second embodiment according to thepresent invention.

FIG. 6 shows a cross sectional view illustrating a relevant portion of aflat tube for heat exchanger in a third embodiment according to thepresent invention.

FIG. 7 shows a perspective view illustrating a state of use of arelevant portion of the flat tube in the third embodiment according tothe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of a flat tube according to the present invention will bedescribed referring to the drawings.

FIG. 1 shows a cross sectional view illustrating a relevant portion of aflat tube according to the present invention; FIG. 2 is a crosssectional view illustrating a state of use of a relevant portion afterbrazing; and FIG. 3 shows a perspective view schematically showing aturned-up portion 4 shown in FIG. 1.

The flat tube for heat exchanger is formed by bending an aluminumstrip-shaped metal plate in the width direction into a nearly B-likeshape in section. As for the strip-shaped metal plate, a brazing sheetis used. In the brazing sheet, the outer surface side of the core metalis coated with a brazing metal of an aluminum alloy up to approximately10% of the total thickness of the plate; and the inner surface side ofthe core metal is coated with a sacrificial anode material of analuminum alloy up to approximately 10% of the total thickness of theplate. The total thickness of the strip-shaped metal plate isapproximately 0.15 mm to 0.6 mm.

The flat tube 8 is formed in a cylindrical shape by a pair of flat faceportions 1 opposed parallelly to each other and a pair of curvedportions 2 with which both ends of the flat face portions 1 areconnected. And in the central portion in the width direction of one flatface portion 1, a turned-up portion 4, which is bent up toward theopposed flat face side, is formed.

Both end edge portions 9 and 10 of the strip-shaped metal plate areoverlapped with each other. One end edge portion 10 is formed in astepped shape, and on the outer surface thereof, the inner surface ofthe end edge portion 9 abuts. The inner surface of the one end edgeportion 10 abuts on the top portion 5 of the turned-up portion 4.

As shown in FIG. 1 and FIG. 3, in the top portion 5 of the turned-upportion 4, many slits 6 for allowing a brazing metal to entertherethrough are formed intermittently being separated away from eachother in the longitudinal direction. The above slits 6 are formed in thefollowing manner. That is, in a state of the strip-shaped metal platebefore bending, the slits 6 are formed as shown in FIG. 4; and then thestrip-shaped metal plate is bent back at the slits 6 as the center.Here, the length “c” of the slits 6 is 2 mm to 15 mm. Also, the distance“e” between the edges of the neighboring slits 6 is 3 mm to 10 mm; and“e/c” is 0.6 or more.

FIG. 5 shows a cross sectional view of another flat tube for heatexchanger according to the present invention. The point different fromthe tube shown in FIG. 1, both end edge portions 9 and 10 of thestrip-shaped metal plate are formed parallelly with respect to theturned-up portion 4; and the end edge portion 9, the end edge portion 10and the turned-up portion 4 are overlapped with each other in thelongitudinal direction of the tube section.

The slits 6 formed in the top portion 5 of the turned-up portion 4 areidentical to those shown in FIG. 1 and FIG. 3.

FIG. 6 shows still another embodiment according to the presentinvention. In this embodiment, a turned-up portion 4 and a turned-upportion 4 a are formed respectively by bending in the central portion inthe width direction of a pair of flat face portions 1 being opposed toeach other, and the respective top portions abut on each other. And inthe top portion 5 of the turned-up portion 4, slits 6 are formed. Thelength of the slits 6 and the distance therebetween are identical tothose shown in FIG. 1. In this embodiment, one end edge portion 9 andthe other edge portion 10 of the strip-shaped metal plate are overlappedwith each other at an end portion of the flat tube 8. In theabove-described embodiments, a single turned-up portion 4 is formed toform a single partition. However, two or more turned-up portions may beformed to form a plurality of partitions. Further, the joint structureof the partition may employ another mode. However, in the presentinvention, it is limited to the structure in which many intermittentslits 6 are formed in the top portion of the turned-up portion 4.

Many flat tubes as described above are disposed parallel to each otheras shown in FIG. 7, fins 7 are disposed between the respective flattubes 8, and both ends of the respective flat tubes 8 are inserted intotube insertion holes in tube headers (not shown) respectively. In astate that the heat exchanger has been assembled, the entire of the heatexchanger is placed in a high temperature furnace to melt the brazingmetal on the outer surface of the flat tube 8, and then cool down thesame to solidify; thereby the flat tubes 8 and the fins 7, the flattubes 8 and the tube insertion holes are integrally fixed by means ofbrazing. At the same time, one end edge portion 9 and the another endedge portion 10 of the flat tubes 8 itself, and the top portion 5 of theturned-up portion 4 and the inner surface abutting thereon areintegrally fixed by means of brazing.

Referring to FIGS. 2 and 3, when the brazing metal 3 is melted in thefurnace, the brazing metal at the outer surface side of the tube entersinto the top portion 5 of the turned-up portion 4 from the slits 6, andcontinuously fixes the top portion 5 and the inner surface of the tubeabutting therewith in a brazing manner. Simultaneously, the outersurfaces of the overlapped turned-up portion 4 are also brazedintegrally.

(Verification of Numerical Limitation in the Present Invention)

In the flat tube 8 according to the present invention, many slits 6 forallowing the brazing metal to enter therethrough are formedintermittently being separated away from each other in the longitudinaldirection of the top portion 5. The length “c” of the slits 6 is 2 mm to15 mm; the distance “e” between the edges of the neighboring slits 6 is3 mm to 10 mm; and “e/c” is 0.6 or more. The above values were obtainedas the optimum values in the present invention on the basis of thefollowing experiments.

As the samples for the experiments, flat tubes 8 shown in FIG. 1 wereformed. The longer diameter of the section was 24 mm; and the shorterdiameter thereof was 2 mm. The thickness of the plate was 0.2 mm, 0.3 mmand 0.4 mm respectively. TABLE 1 Slit length C Gap e Brazing Work-Sample (mm) (mm) performance ability e/c Judgment  (1) 2 3 ◯ ◯ 1.5 ◯ (2) 2 5 ◯ ◯ 2.5 ◯  (3) 2 8 ◯ ◯ 4.0 ◯  (4) 2 10 ◯ ◯ 5.0 ◯  (5) 4 3 ◯ ◯0.75 ◯  (6) 4 5 ◯ ◯ 1.25 ◯  (7) 4 8 ◯ ◯ 2.0 ◯  (8) 4 10 ◯ ◯ 5.0 ◯  (9) 85 ◯ ◯ 0.63 ◯ (10) 8 10 ◯ ◯ 1.25 ◯ (11) 12 8 ◯ ◯ 0.67 ◯ (12) 12 10 ◯ ◯0.83 ◯ (13) 15 9 ◯ ◯ 0.6 ◯ (14) 15 10 ◯ ◯ 0.66 ◯◯: acceptableX: unacceptable

TABLE 2 Slit length C Gap e Brazing Work- Sample (mm) (mm) performanceability e/c Judgment (15) 1 1 X X 1.0 X (16) 1 3 X ◯ 3.0 X (17) 1.5 3 X◯ 2.0 X (18) 1.5 6 X ◯ 4.0 X (19) 2 2 ◯ X 1.0 X (20) 2 12 X ◯ 6.0 X (21)2 20 X ◯ 10.0 X (22) 4 2 ◯ X 0.5 X (23) 4 12 X ◯ 3.0 X (24) 4 20 X ◯ 5.0X (25) 8 2 ◯ X 0.25 X (26) 8 4 ◯ X 0.5 X (27) 8 12 X ◯ 1.5 X (28) 8 20 X◯ 10.0 X (29) 12 2 ◯ X 0.16 X (30) 12 5 ◯ X 0.42 X (31) 12 7 ◯ X 0.58 X(32) 12 12 X ◯ 1.0 X (33) 12 20 X ◯ 1.67 X (34) 15 2 ◯ X 0.13 X (35) 155 ◯ X 0.33 X (36) 15 8 ◯ X 0.53 X (37) 15 12 X ◯ 0.8 X (38) 15 20 X ◯1.33 X (39) 17 5 ◯ X 0.29 X (40) 17 10 ◯ X 0.58 X (41) 17 15 X ◯ 0.88 X(42) 17 20 X ◯ 1.18 X (43) 20 5 ◯ X 0.25 X (44) 20 10 ◯ X 0.5 X (45) 2015 X ◯ 0.75 X (46) 20 20 X ◯ 1.0 X◯: acceptableX: unacceptable

The thickness of the brazing metal 3 coated on the respective outersurfaces was 10% of the total thickness of the plate. As shown in table1, as for the flat tubes according to the present invention, variousflat tubes of which slit length “c” was 2 mm to 15 mm were formed, andprepared so that the length (gap) “e” between the edges of the slits was3 mm to 10 mm; and “e/c” was 0.6 or more.

Also, as the samples for comparison, as shown in table 2, varioussamples other than the flat tubes according to the present invention, ofwhich slit length “c” was 1 mm to 20 mm, were formed and prepared sothat the edges length (gap) “e” between the slits was 1 mm to 20 mm.

The length of the tubes for experiments was 60 mm. The tubes were placedin a high temperature furnace to melt the brazing metal and then cooleddown. The state of the brazing was examined.

As demonstrated in table 1 and table 2, in the aspect of the brazingperformance, satisfactory results were obtained in the following range;i.e., slit length “c” was 2 mm to 20 mm; and the distance “e” betweenthe edges of slits was 2 mm to 10 mm. That is, fillet of brazed pointhaving a satisfactory strength was formed entirely in the top portion 5of the turned-up portion 4 and the performance against the pressure wasensured.

Contrarily, in the cases that the slit length “c” was 1 mm or 1.5 mm,the brazing metal failed to enter satisfactorily through the slits anddefective brazing was found. In the case where the distance “e” betweenthe edges of slits was larger than 10 mm, it was found that a portionwithout fillet of brazed point (not brazed portion) exceeded ⅓ of thedistance “e” between the edges, and the total strength of the flat tubewas not satisfactory. The reason of the above is as described below.That is, in the portion with no slit between the edges, the fillet ofbrazed point was formed with the brazing metal, which entered throughthe slits while brazing, and the length of the entered metal wasconstant. Accordingly, when the distance between edges is too large, alarge portion having no fillet of brazed point was made resulting in areduced strength.

The same results as the above were obtained in any of the followingcases; i.e., the thickness of the plate of the tube was 0.2 mm, 0.3 mmor 0.4 mm.

In the aspect of the workability of the tube, as shown in tables 1 and2, the length of the slits has to be 15 mm or less; and the distance “e”between the edges of slits has to be 3 mm or more; and “e/c” has to be0.6 or more. When the above ranges are exceeded, cracks or twists aregenerated between the edges of slits while the flat tube is being formedand is not suitable to be used as the flat tube. That is, when thelength of the slits exceeds 15 mm, cracks or twists are generated whilethe flat tube is being formed. Also, when the distance between the edgesof slits is 2 mm or smaller, cracks are generated while the flat tube isbeing formed. Further, when the “e/c” is smaller than 0.6, cracks aregenerated while the flat tube is being formed.

The same results as the above were obtained in any of the followingcases; i.e., the thickness of the plate of the tube is 0.2 mm, 0.3 mm or0.4 mm.

Accordingly, it was experimentally verified that the optimum conditionsthat satisfy both of the brazing performance and the workability are asbelow; i.e., the slit length “c” is 2 mm to 15 mm; the distance “e”between the edges of slits 6 is 3 mm to 10 mm; and “e/c” is 0.6 or more.

1. A flat tube for heat exchanger, comprising: a pair of flat faceportions parallelly opposed to each other and a pair of curved portionsconnected to both ends of the flat face portions formed with astrip-shaped metal plate bent in the width direction thereof to forminto a flat cylindrical shape, wherein the strip-shaped metal plate iscoated with a brazing metal on one surface thereof, and is bent so thatthe brazing metal is positioned at the outer surface side of thecylindrical shape; in the central position in the width direction of oneof the flat face portions, a turned-up portion is bent up to the opposedflat surface side, and the top portion of the turned-up portion abuts onthe inner surface of the opposed surface side to form a partition withinthe tube; many slits for allowing the brazing metal to entertherethrough are formed intermittently being separated away from eachother in the top portion in the longitudinal direction thereof, whereinthe length “c” of the slit is 2 mm to 15 mm; the distance “e” betweenthe edges of the neighboring slits is 3 mm to 10 mm; and “e/c” is 0.6 ormore.
 2. The flat tube for heat exchanger according to claim 1, whereinthe thickness of the strip-shaped metal plate is 0.15 mm to 0.6 mm.